1. Field of the Invention
The invention relates generally to using information about errors for improving extracted data sensed from stored data and more specifically relates to apparatus for using information about the extent of errors wherein a transducer is used to sense stored data having at least one constraint from predetermined storage locations. The transducer generates a signal containing at least one constraint and any errors introduced into the sensed data during the sensing. An input device, which in the preferred embodiment is a detector, is responsive to the signal from the sensor to generate a control signal containing information about the extent of errors and extracts a data signal. An output device is responsive to the control signal to perform a control function to effectively improve the extracted data signal as a function of the extent of errors in the sensed data. In addition, the transducer may have at least two sensors or two transducers may be used to produce a first signal and second signal which is applied to the input device to extract a first data signal and a second data signal wherein the output device is responsive to the control signal and to the first data signal and second data signal to derive a data signal therefrom having the least number of  fewer errors.
The apparatus accomplishes this control function by: (i) using the control signal to produce a dynamic servo signal, which in the preferred embodiment is in the form of a substantially continuous servo signal, to adjust a transducer or transducers position to improve alignment of the first transducer relative to predetermined storage locations, which in the preferred embodiment is a track; (ii) deriving, from data signals extracted from signals sensed from two or more transducers or sensors, a data signal having the least number of  fewer errors; or (iii) a combination of producing a dynamic servo signal and deriving a data signal having the east number of errors.
In the preferred embodiment, the input device or detector generates a position error signal from the signal received from the transducer and that position error signal is used as a servo signal for adjusting the transducer position to improve alignment of the transducer relative to the predetermined storage locations.
This invention also relates t a method for using information about errors for performing a control function to improve the extracted data from the stored data in the predetermined storage locations as a function of the extent of errors in the sensed data.
In the preferred embodiment, the invention relates to the field of reading or reproducing from and writing data to mass storage devices, such as magnetic discs.
Also, in the preferred embodiment, the apparatus and method improves the extracted data that is read or reproduced by deriving from among two or more data signals by deriving therefrom the data signal having the least number of  fewer errors, or from a combination thereof. This invention is based on the principle that when at least one constraint is added to the data, this constitutes prior knowledge of the expected data signal in the absence of noise or other impairments. Such knowledge can be exploited, such as for example, by making a computation or comparison between the expected data signal including the at least one constraint and the observed data signal including the at least one constraint, to develop a control signal representing the extent of errors introduced into the signal sensed from the predetermined storage locations by the transducer, transducers or sensors. The so developed control signal can be used to develop a dynamic servo signal in the form of a substantially continuous position error signal which is use to improve head/track alignment using conventional apparatus and methods.
In another embodiment of this invention, by reading and calculating the position error signal on a substantially continuous basis, an effective servo sampling rate is achieved that is much higher than the sampling rates of the state-of-the art apparatus and method.
In the writing mode using a magnetic head, the recorded data is read and the head position is adjusted using the servo system up to the instant the head begins to write thereby improving the quality of the written data on a track.
In yet another embodiment of the present invention, the magnetic head or transducer may contain more than one read sensor. By using the teachings of this invention, the signal representing the extent of errors ay be used to derive a data signal containing the least number of  fewer errors from the more than one data signals.
In a further embodiment of this invention, deriving the data signal which contains the least number of  fewer errors and adjusting the head position to improve alignment with the predetermined storage locations in response to a control signal representing the extent of information about errors may be used in combination to improve the extracted data signal.
2. Description of the Prior Art
It is well known in the art t at magnetic discs operate by providing a surface capable of being magnetized in individualized storage locations or in predetermined storage locations. One or more read/write heads are used for interacting with or transducing with the predetermined storage locations for sensing data store at the predetermined storage locations. The magnetized surface is rotated at relatively high speed, thus presenting each predetermined storage location on the surface to the read/write heads or transducers. The use of and operation of transducers for recording and reproducing data from predetermined storage locations from mass storage devices, such as for example a rotating magnetic disc memory s stem, optical memories, magnetic tape and the like is well known in the art.
It is also known in the art to record or store at predetermined storage locations data or information on rotating surfaces, such as for example, magnetic discs and optical discs. The term “predetermined storage cations” as used herein means the individualized area on a surface, such as for example a predetermined or addressable location on a treated surface area in a rotating memory storage device, which contains or stores data or information. In a rotating memory storage device, which utilizes magnetic media as a magnetic recording surface, the data or information is stored as a series of magnetic field transitions on the magnetic recording surface. Such series of magnetic transitions are included in the term “predetermined storage locations”. The predetermined storage locations are generally recorded or stored in tracks on magnetic media.
Data is read from the predetermined storage locations on the magnetic media using a transducer that interacts with the recording media as a write transducer interacted with the magnetic media when the data or information was recorded or stored. In reading or sensing previously written data from a discrete storage location, the transducer, which functions as a read sensor, must be positioned over or aligned with the tracks on the recording surface where the writing transducer has written or stored the data. Typically, th width allocated to a track of recorded or stored data is wider than the width of the actually recorded predetermined storage locations. Further, the width of the sensor used for reading or sensing the stored data has a width that is less than the width of both the track and of the discrete storage location.
In magnetic discs, data is generally disposed on generally circular tracks, each on the surface of the disc and oriented with its center coaxial with the physical disc. The read/write heads, sometimes referred to as read/write transducers, are disposed in a head/arm assembly so as to be adapted to be moved to a selected track, under control of a disc servo controller.
The state-of-the-art rotating memory systems store as much data as possible on the magnetic disc. This, in turn, requires that the individuated locations or predetermined storage locations of data or information be as small as possible. Further this requires that the tracks on the magnetic media be as close together as possible.
The sensor or transducer for sensing or reading the stored data from predetermined storage locations on magnetic media must be positioned in alignment with the track during the sensing or reading process. The sensors or transducers are operatively connected to arm assemblies, which are sometimes called head/arm assemblies in rotating magnetic disc storage systems, which are adjustable to cause the sensor or transducer to be positioned within the width of the predetermined storage locations containing the stored or written data. If the transducer is positioned within the width of the discrete storage location, the transducer is in precise alignment with the predetermined storage locations and senses or reads the stored data to produce a signal from the sensor with the optimum signal-to-noise ratio and with low data errors.
It is known in the art that, as the track density increases, it becomes more difficult to precisely align the read/write heads or sensors with the tracks. Such misalignments or variances in alignment may occur during reading and writing of data due to variances in operating conditions. The read/write heads or sensors may vary from precise alignment with the tracks by any one of: (i) horizontal displacement from the actual track, (ii) angular orientation or skewing relative to the actual track, or (iii) vertical displacement from the actual track due to the read/write heads or sensor lifting off of the disc. As a result of any of the above variances in operating conditions, the read/write heads or sensor may not precisely be aligned with the track, and such misalignment or variances in alignment change with time.
As a result of such misalignment, the transducing operation of the read/write head or sensors in reading or recovering data from the predetermined storage locations becomes degraded. In addition, the sensed data contains errors or the data sensed by the sensor becomes deformed which results in errors in the electrical signal representing the data or information.
Typically, the transducers were positioned over the rotating magnetic media. Initially, steppe motors were used as actuators to move the sensors to a specific position and the sensors remained in that position during reading and writing of data. This is referred to herein as the “Stepper Motor Method”
One disc drive system which used a process for determining the center of data disc tracks is disclosed in U.S. Pat. No. 4,816,938. In U.S. Pat. No. 4,816,938, a transducer head is positioned to one side of a track. The transducer head repeatedly reads the track and is microstepped across the track. The number of microsteps taken at the position on each side of the center of the rack, where a certain number of error corrections occur, are designated as the boundaries of that track. The center of the track is then calculated as being half-way between the boundaries. By using the microstep off-sets for the centers of two tracks, a correction factor can be calculated to compensate for thermal expansion of the disc.
Another method known in the art for controlling transducer head/track alignment is to use a closed loop servo system having a dedicated servo surface. In this method, continuous sequences of special positioning signals were recorded on the servo surface in every track on the servo surface. Deviation signals were developed using the prerecorded sequences and such deviation signals were used in a feedback technique to adjust the position of all of the other read/write transducer heads. This is referred to herein as the “Servo Surface Method”
Another known method includes “servo bursts” being prerecorded around each track to improve the alignment of the data head with the data track. A “servo burst” is a short special position signal, i.e., equivalent to the length of few bits, prerecorded in each track, used in a servo system for maintaining head/track alignment.
The prerecorded “servo burst” pattern is presently used for generating a signal indicating the magnitude of misalignment between the data head and track an the direction that the head was displaced from the track center. Special cases provide for prerecording a “servo burst” in each sector of a disc. The so generated signal is used to locate track position more precisely in terms of predefined actuator positions and functioned to place the head in alignment with the track.
The recorded servo bursts are used to provide an open loop servo system to more accurately and rapidly position a head relative to a track prior to reading of and writing of data onto and from the track and to adjust or correct head position during the process of reading and writing of data. This is referred to herein as the “Servo Burst Method”.
U.S. Pat. No. 5,233,487 discloses a rotating media storage system that compensates for thermal and mechanical errors in the position of the data detector, or read head, with respect to the written data. The compensation is accomplished by measuring the error rate of written data as a function of the read offset of the detector in that error rates become increasingly large as the sensed noise to signal ratio becomes large. As the offset of the head becomes misaligned with the track, the sensed noise to signal ratio increases. When the data storage system is initially activated, the detector counts the number of errors detected in reading written data for various read offsets. When the number of errors reaches a target rate, the read offset corresponding to the target rate is saved. The procedure is performed on either side of the data track. During operation of the storage system, thermal and mechanical operational errors occur in the system and similar error rate data and offset data are developed for these operational conditions. The so developed error rate and target error rate are used to cause the detector to be re-centered with respect to the write transducer position between the new offsets on either side of the tracks.
Use of a dual-striped magnet resistive head with a conventional servo system was disclosed in an article entitled Estimation of Track Misregistration by Using Dual-Stripe Magnetoresistive Heads, by Lian N Zhi Gang Wang, Desmond J. Mapps, P. Robinson, Warwick W. Cleg , D. T. Wilton and Yoshihisa Nakamura, which appeared at Pages 2348 to 2355 of the IEEE TRANSACTIONS ON MAGNETICS, Volume 34, No. Jul. 4, 1998 (the “Wang et al Reference”). The Wang et al Reference utilized the principal that when a dual-stripe, unshielded magnetoresistive (“MR”) element is exposed to the same stray field from a media transition, one MR element has a resistance increase and the other MR element has a resistance decrease. The difference between the MR element's output envelopes was demodulated by a peak value detection circuit and the sum of the two element signals was considered as a position error signal. The position error signal was utilized in conjunction with a conventional servo system and the estimated off track perturbation was used to supplement the well used sector servo. This system did not use a constraint within the data or the Extent of errors therein.
U.S. Pat. No. 4,404,676 discloses a method using a data-dependent code word consisting of redundancy bits, that marks a boundary of a multi-bit cell. The data-dependent code word is coded to bear a “mapping relationship” to a data block within the cell. Embodiments using the data-dependent code word provide various types of synchronization. Decoders provide block and bit synchronization for either serial-by-bit data signal or a serial-by-byte data signal. U.S. Pat. No. 4,404,676 further discloses that the preferred embodiment is used in a record/playback system for storing on a storage medium and subsequently deriving the stored information from the storage medium. In the system disclosed in U.S. Pat. No. 4,404,676, encoding means and decoding means are used, each of which utilizes a data-dependent boundary-marking code word bearing a predetermined mapping relationship to a data block of a cell and otherwise being indistinguishable from arbitrarily selected groups of data bits. The object of U.S. Pat. No. 4,404,676 is to solve synchronization problems and not to provide for adjusting head or transducer position or deriving a data signal having the least amount of errors from two or more data signals representing the sensed data.
As is well known in the art, a servo system in rotating memory systems is used to control the positioning of the sensor relative to the tracks of predetermined storage locations containing the stored data. Servo systems, which are well known in the art, include sector servo systems, dedicated servo systems or other well known servo systems. The function of the servo system is to compare certain sensed signals with a predetermined pattern of signals and to use the results of such a comparison to change position of the sensor or transducer with respect to the predetermined storage locations to generate the most accurate reading of the stored data from the predetermined storage locations.
The known prior art servo systems and methods for sensing and providing efficient and rapid adjustments of the sensing head have many disadvantages.
In the Stepper Motor Method, no feedback signals were used to adjust head position.
In the system and method disclosed in U.S. Pat. No. 4,816,938 reading of the data or writing of new data must occur in an open loop system, e.g. no servo loops are used, during reading and writing of the data. Error correction codes were used only to help select transducer head position prior to reading or writing of the data.
With respect to the Servo Surface Method as discussed hereinbefore, while this method partially achieves the goal of attempting to align the read/write head with the data track, it has at least the following drawbacks. First, this method uses substantial disc space for the servo sequences, which removes area from the disc that could otherwise be used for data. Second, the Servo Surface Method results in misalignment between the servo head and the data head and is not practical for use in magnetic disc drives having higher areal densities. While the Servo Surface Method as augmented by U.S. Pat. No. 5,233,487 provides a correction for changes in the relation between the servo surface head and data head due to thermal expansion and other factors, servoing during data reading and writing is restricted to the servo surface.
The Servo Burst Method as described above is an intermittent servo system that runs open loop between servo bursts and closed loop upon the sensing of the recorded servo burst to produce servo signals for adjusting the head position relative to the data track. The Servo Burst Method is generally known in the industry as open loop technology and is the standard today of the industry for magnetic storage systems. This method, even as augmented by U.S. Pat. No. 5,23,487 and the Wang et al reference, has at least the following drawbacks.
First, if it is desired to improve the sampling accuracy, additional disc space for the servo bursts would be required. Second, the accuracy of this method must be significantly improved upon for use in the state-of-the art high areal density storage systems
The method disclosed in U.S. Pat. No. 5,233,487 is based on the principal that sensing signals outside of the write width is deemed an off-track read, and when enough of the read widths are outside of the write width, an error in reading data is deemed to have occurred. An ECC detection/correction means senses the error in blocks of data and a counter maintains a count of the number of errors. When the errors reach a predetermined error rate, the data detector position is adjusted to optimize detector read performance. As such, adjustment of the read head relative to the data track occurs only after the number of counted errors exceeds a predetermined number of errors.
The use of a dual-stripe MR head, as disclosed in the Wang et al Reference, was based on the principal that the signal waveform changes only in amplitude as the head moves off track, and does not rely on the data signal having any constraints.
The system and method disclosed in U.S. Pat. No. 4,404,676 has several limitations when applied to magnetic data storage systems. During reproduction of the data in such a system, it is necessary to produce a plurality of individually identifiable clock signals and each clock signal has reoccurring clock pulses with the clock pulses of each such clock signal differing from those of each other clock pulses as to the time of occurrence. The block pulses are required to sample a data signal and to derive a plurality of sample bits that are congregated and tested. The system includes means for generating a candidate-valid signal for each candidate cell that is determined, by such testing, to be a valid cell. As each candidate cell undergoes such testing, that testing determines whether the code-word portion of the candidate cell bears the predetermined mapping relationship to its block-length portion as is characteristic of a valid cell. Since this system is based on a serial-by-bit data signal or a serial-by-byte data signal method, the system is not designed and is unable to produce a position error signal, or to derive a signal from among multiple input signals.
In the last several years, the state-of-the art performance benchmark for the magnetic hard disc drive industry has been areal density progression. Areal density is defined as the number of bits per square inch that can be stored on a magnetic disc surface and successfully retrieved. Areal density is determined mathematically by Bits Per Inch (“BPI”) multiplied by Tracks Per Inch (“TPI”) (BPI×TPI). As the areal density increases, the need for an improved head positioning apparatus, methods and systems likewise becomes necessary due to the limitations of the prior art systems, methods and apparatus as described above.
Areal density for magnetic hard disc drives has been increasing at a 60% compounded annual growth rate and that rate is forecasted to continue for at least the next several years. For example, the areal density for magnetic hard disc drives was 1 gigabit per square inch (“Gb/sq. in.”) in 1995 and increased to 4.1 Gb/sq. in. in 1998.
In addition, it is reasonable to expect that at some areal density point, which is presently estimated at about 40 Gb/sq. in., the issue of thermal decay of the stored data on the magnetic media will need to be addressed. It is reasonable to conclude that other storage media, such as for example, chemical and molecular media, may be developed for storage of data in predetermined storage locations.
Increases in TPI will probably depend on the ability of the read head or read transducer to both track the data and to respond mechanically to non-repeatable spindle motor problems and other track misregistration problems.
In 1998, typical BPI is 256,000 and TPI is 16,000 which represent an areal density of about 4.1 Gb/sq. in. For the year 2000, the areal density is forecast to be 10 Gb/sq. in. which corresponds to a BPI of 334,000 and a TPI of 30,000.
It is reasonable to conclude that a track density may exceed 86,000 TPI and, if so, the read head or read sensor will have to follow a written track width of 10 microinches, all of which will require advanced servo systems and data track following technology utilizing the teachings of the present invention.